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  <h1 class="topictitle1">Automatic Chunking</h1>
 
   
  <div> 
	 <p>A parallel loop construct incurs overhead cost for every chunk of work
		that it schedules. Since version 2.2, Intel&reg; Threading Building Blocks (Intel&reg; TBB) chooses chunk sizes
		automatically, depending upon load balancing needs.<a href="#ftn1"><sup><sup>[1]</sup></sup></a>
		The heuristic attempts to limit overheads while still providing ample
		opportunities for load balancing. 
	 </p>
 
	 <div class="Note"><h3 class="NoteTipHead">
					Caution</h3> 
		<p>Typically a loop needs to take at least a million clock cycles to make
		  it worth using 
		  <samp class="codeph">parallel_for</samp>. For example, a loop that takes at least
		  500 microseconds on a 2 GHz processor might benefit from 
		  <samp class="codeph">parallel_for</samp>. 
		</p>
 
	 </div> 
	 <p>The default automatic chunking is recommended for most uses. As with
		most heuristics, however, there are situations where controlling the chunk size
		more precisely might yield better performance. 
	 </p>
 
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<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong>&nbsp;<a href="../tbb_userguide/parallel_for.htm">parallel_for</a></div>
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<div class="See Also">
<h2>See Also</h2>
<div class="linklist">
<div><a href="Controlling_Chunking.htm#tutorial_Controlling_Chunking">Controlling Chunking 
		  </a></div></div>
</div> 
<p class="tfootnote"><a id="ftn1"><sup>[1]</sup></a>  In Intel&reg; TBB 2.1, the default
		  was 
		  <em>not</em> automatic. Compile with 
		  <samp class="codeph">TBB_DEPRECATED=1</samp> to get the old default behavior.</p>
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